Using a new electrochemical technique which is light sensitive and catalytic, it will be possible to detect 10 to the minus 17th power to 10 to the minus 18th power moles (10 to the minus 13th power to 10 to the minus 14th power M) of a ruthenium complex. The ruthenium complex will be coupled to thyroid hormones, TSH and TRH to perform photoelectrochemical immunoassay. Initially the photoelectroanalytical system will be studied to determine conditions for maximum signal to noise ratio. This system will be used to detect free photoelectrochemically labeled hapten in the most sensitive non-radioactive immunoassay yet developed.